Direct conversion radio receivers, also referred to as zero-IF receivers, inherently suffer from the problem of DC offsets. Sources of DC offset are present in all stages of a direct conversion receiver due to component imbalances and tolerances. Further sources of DC offset include leakage of the local oscillator signal used in the receiver, for example, conduction and/or radiation of the local oscillator signal to the receiver antenna and leakage of the received signal into the local oscillator port of the RF mixers used in the receiver.
In a conventional direct conversion radio receiver operating, for example, in accordance with the GSM standard, which uses GMSK (Gaussian Minimum Shift Keying) modulation, the received signal is coupled to RF mixers which provide a baseband in-phase signal component I and a baseband quadrature signal component Q.
A hardware solution to the DC offset problem is implemented in direct conversion receivers such as that used in the Nokia 6210™ mobile telephone. The fact that the receiver is intended for use with GSM time division multiple access (TDMA) signals has been exploited to incorporate time periods in which the DC component of the I/Q signals can be clamped to zero by a DC cancellation (DCN) circuit. The timing of the DCN periods is controlled to occur in advance of the reception of any received signal burst. The DCN circuits are designed as high pass filters, in which capacitors can be rapidly charged/discharged during the DCN period by electronic switching circuits, to obtain a subtraction of the DC offset in each I or Q channel. The weakness of this solution is that the precision of the DC compensation is dependent on the RF signal received during the DCN period. Because the DCN period lies in the time slot preceding the time slot allocated for the mobile station's own received signal, the signal in the DCN period can range from OdBm to, for example, more than 30 dB higher than the level in the receive time slot allocated to the mobile station. When the received level in the DCN period is, for example, 30 dB higher than the mobile station's own received signal, the GMSK modulation cannot be averaged sufficiently by the DCN capacitors, resulting in an excessive residual DC content in the I/Q samples of the mobile station's own signal. This in turn results in a poor bit error rate (BER) for the received burst.
The above described problem also prevents direct conversion receivers using the DCN hardware solution from complying with certain tests used in GSM Type Approval, where signals which differ by 20 dB between adjacent time slots are applied to a mobile station.
The present invention aims to address the above problems.